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I haven't been able to find a diagram of a roof with the exact configuration ours has. I think the board on which our joists rest is called the wall plate but what is the name of the board that sits on the joists and on which the rafters rest?

This is in a garage and I think the plan was to make the roof space bigger for a future conversion because windows are already in place and wiring. The reason I'm asking stems from the fact that I want to sister a couple of joists so I can remove part of one for an opening.

enter image description here

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    Normally both the joist and the rafter rest on the "top plate" or the "wall plate". The rater is also normally notched at the top plate. What you have drawn is atypical.
    – jwh20
    Sep 15, 2020 at 11:29
  • That's what I thought too but ours is definitely as in the diagram.The rafter goes nowhere near the wall thoufh it does extend a lot further than I drew so as to form quite a wide soffit.
    – culchie
    Sep 15, 2020 at 11:38
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    I didn't mean to imply that it's wrong to do it this way, it's atypical because it uses more material for no obvious benefit. It's effectively a raised top plate. It's also not clear why the joists are extended. Perhaps the builder just wanted to be different.
    – jwh20
    Sep 15, 2020 at 12:02
  • Hi jwh, this is in a garage and I think the plan was to make the roof space bigger for a future conversion because windows are already in place and wiring. The reason I'm asking stems from the fact that I want to sister a couple of joists so I can remove part of one for an opening. Might I be a bit cheeky and ask if just resting another 2X10 on at least a bit of the "normal" plateon each side of the building but trimmed away so it's not touching the upper plate would be OK. Or does it need to support that as well? I'd be grateful for any advice!
    – culchie
    Sep 15, 2020 at 12:25
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    The home I built in Maryland, that I lived in for 25 years had this type of construction. It was a Cape Cod style home with an 11/12 pitch roof. It was needed for both reasons 2nd floor space and overhang details. I call it a nailer, which those are typically further defined by their purpose, in this case, a nailer for the rafters. Raised plate fits too. Purlin, IMHO is a bit of a stretch since that term is seems to always applied to a member that is applied to the face of studs or rafters or a mid span support under rafters.
    – Jack
    Sep 15, 2020 at 14:23

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I'm not sure there's an official standard name for this specific scenario, but I'd call it a purlin or an elevated wall plate, maybe. A diagram in this discussion has it as a raised top plate. This article discusses purlins as an intermediate rafter support member.

jwh20 is right in that this is a non-standard roof framing method, but there are a few reasons it may have been done (one of which you mentioned--attic space). Another is to create an "energy heel" or "raised heel", where there's more vertical space for insulation. Yet another would be to raise the fascia/soffit height to clear windows or improve appearance (in the case of a very steep pitch). In your case the ceiling joists may have been run through to create backing for the soffit, which is often added later at a lower level.

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